Method and apparatus for producing contrast agents for magnetic resonance imaging

ABSTRACT

The present invention relates to an arrangement and a method for providing contrast agent for e.g. MRI (Magnetic Resonance Imaging) and NMR (Nuclear Magnetic Resonance) applications. The method according to the invention comprises the steps of obtaining ( 100 ) a solution in a solvent of a hydrogenatable, unsaturated substrate compound and a catalyst for the hydrogenation of a substrate compound, hydrogenating ( 110 ) the substrate with hydrogen gas (H 2 ) enriched in para-hydrogen (p- 1 H 2 ) to form a hydrogenated contrast agent and exposing ( 120 ) the contrast agent to a oscillating magnetic field in combination with a stationary magnetic field. The apparatus comprises a magnetic treatment unit ( 240 ) equipped with means for producing an oscillating and a stationary magnetic field.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method forpara-hydrogen induced hyperpolarization of a compound, and in particularfor the preparation of a contrast agent for magnetic resonance imagingprocedures.

BACKGROUND OF THE INVENTION

Magnetic Resonance Imaging (MRI is an important diagnostic technique. Itis especially attractive since it is non-invasive and does not exposethe patient to potentially harmful radiation such as X-rays or radiationfrom radioactive materials. Significant progress has recently been madein the quality of images as well as finding new applications of thetechnique. The progress relies on a rapid development of the digitalimage processing, the refined Nuclear Magnetic Resonance (NMR)techniques and the development of effective contrast agents (imagingagents). An emerging technique of particular interest involves MagneticResonance (MR) contrast agents based on the principle ofpre-polarization of the nuclear spins, also called hyperpolarization

For the resonance phenomena, which is the basis of NMR and MRI, tooccur, isotopes with non-zero nuclear spin have to be present. Inaddition, since NMR is not an extremely sensitive technique, arelatively high concentration and/or a high gyromagnetic ratio isneeded, especially for imaging purposes. The use of a selected isotopein a contrast agent which is to be injected in a patient, puts furtherrequirements on the selected isotope, for example regarding toxicity. Anumber of isotopes have the required spin properties, but only a few areconsidered interesting for the use in contrast agents, for example thecarbon isotope ¹³C and the nitrogen isotope ¹⁵N. The carbon isotope ¹³Chas many properties that would it make it useful as a functional part ofan MRI contrast agent. An important feature is the long longitudinalrelaxation time, T₁. The relaxation time needs to be long in order tohave time, after the generation of the contrast agent, to inject thecontrast agent into the patient and to allow the contrast agent to betransported to the organ that is to be studied. To make a usefulMR-contrast agent of this kind, the signal strength has to be boostedsignificantly over the thermal equilibrium signal. Patent application WO00/71166, by the same applicant, describes a process and an apparatusfor increasing the polarization and hence the signal from a smallorganic molecule containing for example ¹³C. The signal was increasedwith a factor 10⁴. The process is referred to as Para-Hydrogen InducedPolarisation (PHIP) and may comprise the transferring of nuclearspin-order from para-hydrogen to spin polarization in non-zero spinnuclei in molecules, e.g. to ¹³C or ¹⁵N nuclei.

Hydrogen molecules exist in four different spin states. In one of theforms, characterised by antiparallel spins, the magnetic moments of theprotons cancel. This form is called para-hydrogen. The other threeforms, with a net magnetic moment, are referred to as ortho-hydrogen.The para-hydrogen will not rotate at low temperature whereas theortho-form must rotate with a high frequency at all temperatures becauseof quantum-mechanical symmetry requirements of the wave function. Thisindicates that at low temperature the para-form will have asignificantly lower energy, and hence is the energetically favouredform. At temperatures below 20 K the equilibrium ratio of para- andortho-hydrogen approaches 100:0, at 80 K the ratio is 48:52 and at roomtemperature approximately 1:3. The equilibration can be speeded up bythe presence of a transition metal catalyst, e.g. Fe₂O₃. Para-hydrogenrelaxes slowly (if no catalyst is present) at room temperature.

In WO 00/71166 it is described how to catalytically hydrogenate (withpara-hydrogen) unsaturated compounds comprising non-zero spin nucleisuch as ¹³C. The spin correlation of the protons from the para-hydrogenwill be preserved during and after hydrogenation, and the influence onthe spins of the ¹³C nuclei breaks the symmetry of the spin system. Theprotons will now give an NMR-signal, but the non-equilibrium spin orderis not sufficient to make the molecule useful for imaging purposes sinceit has an anti-phase behaviour that is not ideal for imaging. In theabove cited applications, and further in “Parahydrogen-InducedPolarization in Imaging” by K. Golman et al., Magnetic Resonance inMedicine 46:1-5 (2001), a magnetic field cycling method is described fortransforming the proton spin-order to carbon spin polarization. In afirst step the external magnetic field is reduced (from the relativelyhigh geomagnetic field) bringing the combined proton-carbon spin systeminto its strong coupling regime. In this regime the scalar coupling(J-coupling) strongly influences the evolution of the spin system. Thereduction of the field should be fast, giving a diabatic (non-adiabatic)process. In a subsequent step the field strength is slowly increased (anadiabatic process). The field cycling will result in a substantialincrease in the polarization of the spins of the ¹³C nuclei, giving anin-phase NMR-signal, and increase the usefulness of the compound as acontrast agent for use in imaging procedures. However, the result of thefield treatment will be dependent on the scalar coupling of the combinedspin system and the properties of the magnetic field. In WO 00/71166examples of field cycling schemes are described that give a substantialimprovement in the image quality. To make the method even moreattractive for use in medical and diagnostic applications it would be ofhigh value to further increase the degree of polarisation of the carbonspins and to prolong the relaxation time of the nuclear spin system.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and anapparatus for producing MRI contrast agent with a high degree ofpolarisation of the imaging nuclei spins.

The object is achieved by the method as defined in claim 1, theapparatus as defined in claim 11, and by the computer program product asdefined in claims 17 and 18.

The method for producing contrast agent according to the presentinvention comprises the steps of obtaining a solution in a solvent of ahydrogenatable, unsaturated substrate compound and a catalyst for thehydrogenation of a substrate compound, hydrogenating the substrate withhydrogen gas (H₂) enriched in para-hydrogen (p-^(l)H₂) to form ahydrogenated contrast agent and exposing the contrast agent to aoscillating magnetic field in combination with a stationary magneticfield for enhancing the contrasting effects of the contrast agentadapted for use in an MR application.

According to one embodiment of the present invention the oscillatingmagnetic field is oscillating with a frequency within the region ofradio frequencies (around 10 Hz to several GHz) and preferably with afrequency in the interval 5 kHz to 500 MHz.

According to a further embodiment of the present invention the step ofexposure to the oscillating magnetic field in combination with thestationary magnetic field is performed during the step of hydrogenation,wherein the step of exposure is performed for reducing the relaxation ofthe spin system of the contrast agent, whereby the contrasting effectsof the contrast agent is enhanced.

According to a still further embodiment of the invention the step ofexposure to the oscillating magnetic field in combination with thestationary magnetic field is performed after the step of hydrogenation,the step of exposure is performed for enhancing the degree ofpolarization of imaging nuclei of the contrast agent, whereby thecontrasting effects of the contrast agent is enhanced. The oscillatingfield is applied in the form of a series of pulses with the frequencyand angle varying and adapted for the contrast agent in use.

The apparatus for producing MR contrast agent according to the presentinvention comprises means for producing an oscillating magnetic fieldand means for producing a stationary magnetic field. The means forproducing both the oscillating magnetic field and the stationarymagnetic field may advantageously be of the same type as commonly usedin NMR equipment.

One advantage afforded by the apparatus and method according to thepresent invention is that contrast agent can be produced thatsignificantly improves the image quality and/or speeds up the process inan MRI application and/or improves the analysis performance in an MNRapplication.

A further advantage is that novel types of imaging, not possible, orvery difficult to perform with prior art techniques, can be performed.

The advantages are achieved by the apparatus and method according to thepresent invention by providing a high degree of polarization of theimaging nuclei of the contrast agent, by that the process of producingcontrast agent is fast, thereby reducing the problems with relaxation ofthe spins system of the contrast agent and by that the relaxation of thespins system is effectively reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to thedrawing figures, in which

FIG. 1 is a flowchart illustrating the main steps of the methodaccording to the invention;

FIG. 2 is a schematic drawing illustrating the apparatus according tothe invention;

FIG. 3 is a schematic drawing illustrating the magnetic treatment unitof the apparatus according to the invention is;

FIG. 4 is a flowchart illustrating one embodiment of the methodaccording to the invention;

FIG. 5 is a flowchart illustrating one embodiment of the methodaccording to the invention;

DETAILED DESCRIPTION OF THE INVENTION

Parts of apparatus and parts of the process described in WO 00/71166 areadvantageously utilised also in the present invention. WO 00/71166teaches that the hydrogenation reaction is preferably performed bymixing gaseous para-hydrogen (or ortho-deuterium) enriched hydrogen witha solution of an unsaturated compound and a hydrogenation catalyst.

The main steps of the of the method according to the invention will bedescribed with reference to the flowchart of FIG. 1 and comprises thefollowing main steps:

-   100: obtaining a solution of a solvent, a hydrogenatable,    unsaturated substrate compound and a catalyst for the hydrogenation    of a substrate compound;-   110: introducing the solution into a chamber containing hydrogen gas    (H₂) enriched in para-hydrogen (p-¹H₂) in order to hydrogenate the    substrate to form a hydrogenated contrast agent.

The method according to the present invention introduces a main step of:

-   120: subjecting the contrast agent to an oscillating magnetic field    in combination with a stationary magnetic field.;

The oscillations of the oscillating magnetic field should preferably bein the kHz-MHz-region. Such oscillating fields are commonly known asrf-fields, and the term rf-field will hereafter be used to denote anoscillating magnetic field with a frequency in the interval 5 kHz to 500MHz. A frequency range of 10-500 kHz is preferred for many applications.The rf-field treatment enables spin-order to be transferred from protonsin the freshly hydrogenated contrast agent into polarization of anucleus within the same molecule with a slower relaxation, preferably a¹³C or ¹⁵N nuclei. These nuclei will be referred to as imaging nuclei.In addition an rf-field may be used to reduce the relaxation rate of thespin systems during the hydrogenation-process, giving a higherpolarization of the contrast agent. An apparatus and a method accordingto the present invention for providing the rf-field and stationary fieldduring the production of the imaging agent is described below. Themethod and apparatus will be exemplified with contrast agent comprisingcarbon (¹³C). This should be regarded as a non limiting example. Otherisotopes could be used with slight modifications to the detailed stepsof the method.

The hydrogenatable substrate used may be a material such as apara-hydrogenation substrate as discussed in WO99/24080. For in vitro orin vivo MR studies of biological or quasi-biological processes orsynthetic polymer (e.g. peptide, polynucleic acid etc.) syntheses, thesubstrate is preferably hydrogenatable to form a molecule participatingin such reactions, e.g. an amino acid, a nucleic acid, areceptor-binding molecule, etc., either a natural such molecule or ananalog.

The solvent used in step 100 of the process of the invention may be anyconvenient material which serves as a solvent for the substrate and thehydrogenation catalyst. A number of possible solvents are discussed inWO99/24080. When the contrast agent is for use in in vivo MRinvestigations, the solvent is preferably physiologically tolerable.Water is a preferred choice of solvent, used in combination with awater-soluble catalyst. If other solvents that are not physiologicallytolerable are used, the solvents has to be removed before use in apatient, for example by vacuum-spray. Other rapid solvent removaltechniques, e.g. affinity techniques, may however be used. The solventis preferably used at or near the minimum quantities required tomaintain substrate, catalyst and contrast agent in solution withoutexperiencing viscosity problems during the hydrogenation reaction.

The hydrogenation catalyst is preferably a catalyst as discussed inWO99/24080, e.g. a metal complex, in particular a rhodium complex.

The enriched hydrogen, which may be pure ¹H₂ or ²H₂, or a mixture of ¹H₂and ²H₂, may optionally contain other gases, but is preferably free fromoxygen or other reactive or paramagnetic gases, may be prepared bycooling hydrogen, preferably to a temperature below 80 K, morepreferably to below 50 K, even more preferably to below 30 K andespecially preferably to below 22 K, and allowing the nuclear spinstates to equilibrate, optionally in the presence of a solid phaseequilibration promoter, e.g. Fe₃O₄, Fe₂O₃, activated charcoal, etc. Theenriched hydrogen is then preferably removed from the equilibrator andoptionally stored before use. A method of preparation and storage ofenriched hydrogen is described in WO99/24080. A preferred and novelmethod of storage and equipment for that purpose has been developed bythe inventors. The enriched hydrogen is transferred to and stored in gascylinders made of inert material. Inert in this context should beunderstood as made up by material essentially free from paramagneticmaterials (primarily iron) and other para-hydrogen relaxing compounds(e.g. palladium). Examples of inert materials suitable for the gascylinders are aluminum and carbon fiber reinforced epoxy. The enrichedhydrogen will decay slowly (in the order of 10% per week). Such a decayrate is acceptable for most applications, especially compared with thecost and handling problems of previous storing methods involving storingat cryogenic temperatures.

For the hydrogenation reaction, a reaction chamber is filled withenriched hydrogen optionally under pressure, preferably 5-20 bar, andthe catalyst and substrate solution is introduced either as a thin jet,by spraying or by atomising, into this reactor. If desired, the solutionmay be produced by mixing separate solutions of catalyst and ofsubstrate. To ensure proper mixing, a distributor or a plurality ofspray nozzles may be used and the chamber contents may be mixed, e.g. bya mechanical stirrer or by appropriately shaping the chamber walls topromote turbulent mixing where there is a flow of reaction mixture inthe chamber.

The process may be performed continuously with a flow reactor, e.g. aloop or tube reactor, or alternatively it may be a batch-wise process.Preferably there will be a continuous or pulsed flow of enrichedhydrogen and solution into the reactor, a continuous or batch-wiseremoval of liquid solution from the base of the reactor, and acontinuous or batch-wise venting of unreacted gas from the reactor. Theenriched hydrogen and solution passing into the reactor are preferablytemperature-controlled to ensure the gas/droplet phase in the reactor isat the desired temperature. This can be achieved by providing inputlines with temperature sensors and heating and/or cooling jackets.

If a non-aqueous solution has been used the contrast agent is preferablymixed with water after the hydrogenation and the exposure to themagnetic fields. The water used is preferably sterile and alsopreferably essentially free of paramagnetic contaminants. The resultantaqueous solution is then preferably treated to remove the hydrogenationcatalyst, e.g. by passage through an ion exchange column, preferably onefree of paramagnetic contaminants. The water may betemperature-controlled as may be a mixing chamber where water andcontrast agent solutions are mixed so as to ensure the aqueous solutionenters the ion exchange column at the appropriate temperature. Stronglyacidic, sodium ion charged ion exchange resins such as DOWEX 1x2-400(DowChemicals) and Amberlite IR-120 (both available from Aldrich Chemicals)resins may conveniently be used for the removal of typical metal complexhydrogenation catalysts. For fast ion exchange, the resin is preferablycross-linked to only a low degree, e.g. a 2% divinyl benzenecross-linked sulphonated, sodium ion loaded polystyrene resin.

Removal of the non-aqueous solvent may then conveniently be effected byspray flash distillation e.g. by spraying the aqueous solution into achamber, applying a vacuum, and driving the organic solvent free aqueoussolution from the chamber using an inert, preferably non-paramagneticgas, e.g. nitrogen. Indeed in general the flow of liquid componentsthrough the hydrogenation apparatus is preferably effected using appliednitrogen pressure, e.g. 2 to 10 bar.

The resulting aqueous contrast agent solution may be frozen and storedor may preferably be used directly in an MR imaging or spectroscopyprocedure, optionally after dilution or addition of further solutioncomponents, e.g. pH modifiers, complexing agents, etc. Such direct usemay for example involve continuous infusion or alternatively injectionor infusion of one or more dose units. Bolus injection is particularlyinteresting.

The whole process from beginning of hydrogenation to the delivery of thefinished contrast agent in for example a syringe may conveniently beeffected in less than 100 seconds, indeed it is feasible to producedosage units in less than 10 seconds, which is substantially less thanT₁ for the potentially interesting imaging nuclei.

Preferably, the surfaces contacted by the contrast agent during theprocess of the invention are substantially free of paramagneticmaterials, e.g. made of glasses as used for hyperpolarized ³Hecontainment as discussed in WO99/17304 or gold or polymer, optionallydeuterated. Surfaces contacting a non-aqueous solvent (e.g. acetone)should be acetone-resistant and valves may be magnetically controlledand provided with solvent resistant Teflon or silicone parts.

An apparatus suitable for producing contrast agent by the methodaccording to the present invention will be described with reference toFIG. 2. Hydrogen (¹H₂) enriched in para-hydrogen is fed from thepara-hydrogen source 200 into a reactor 210. A hydrogenation catalystsolution from a catalyst reservoir 220 and a hydrogenatable substratesolution from a substrate reservoir 230 are fed into the reactor 210.The liquid settling in reactor 210 is transferred to a magnetictreatment unit 240, which essentially comprises a compartment for thesolution surrounded by a low field, two-channel NMR spectrometer, andthence to finishing unit 250 for cleanup, quality control and possibleaddition of additives and solvent removal. The finishing unit maycomprise an ion exchange column and a solvent removal chamber equippedwith a spray nozzle. After the passage through the finishing units theimaging agent is delivered to, for example, a syringe for injection in apatient Alternatively the imaging agent is stored for later usage. Thereactor 210, may in addition be provided with a initial magnetictreatment unit 260 adapted for producing a stationary magnetic field andan rf-field. As an alternative, the initial magnetic treatment unit 260may be combined with the magnetic treatment unit 240 whereby allmagnetic treatment will be performed within the reactor 210.

An embodiment of a magnetic treatment unit 240 in accordance with thepresent invention will be described with reference to the schematicblock diagram of FIG. 3, in which the main parts of the NMR-unit areoutlined. The same type of NMR-unit may be utilized as the initialmagnetic treatment unit 260, or a combined magnetic treatment unit. TheNMR unit comprises a coil system 300, a radio frequency synthesizer 310,an amplifier 320, and a computer 330. The coils of the coil system 300are connected to power supplies 345, which preferably are connected toand controllable by the computer 330. The coil system comprises aresistive magnet that gives a main field of 0.1-100 mT, preferably 1-10mT. This magnet may preferably be made as a Helmholz pair 340 with alarge radius, around 30 cm, to give a high homogeneity over the samplespace which preferably has a volume of 10-100 mL in the center of thecoil system. The coil system 300 is preferably provided with threelinear shim coils, x, y, and z (not shown) to allow for compensation ofexternal field gradients, which primarily may be due to the presence ofa nearby MRI system. The inhomogeneity of the Helmholtz pair will alsobe improved by the shims. Any other geometry (solenoidal) or magnetdesign (permanent magnet) compatible with the general design of theequipment can be used. Resistive magnets are suitable, due to themoderate field levels required. Superconducting magnets may also beused. Higher order shims can be added if there is need to place thisequipment very close to the MRI magnet. When required, the magnet coilsmay, completely or in part, be wound from aluminum wire to reduceweight. Pairs of Hall plates (not shown) or other magnetic fielddetectors may also be present to allow for measurement of the fieldgradients in x, y and z directions so that the computer 330 cancompensate for field disturbances automatically. The Hall sensorsprovide in addition the mean value of the magnetic field strength and isused to lock the field to the transmitter frequencies.

An NMR-coil 360 is used to produce the excitation signal. The excitationsignal is conveniently generated digitally. The rf-coil 360 is connectedvia switching means 365 and a amplifier 320 to a digital waveformgenerator 375, the amplifier and the waveform generator forming therf-synthesizer 310. The digital waveform generator can be a stand-aloneinstrument or a PC card. The desired frequency spectrum and phaserelations are created arbitrarily within the limits of the timeresolution of the instrument/device. The excitation signal from thewaveform generator is amplified to create the desired excitation fieldstrength in the coil. Generally, the pulses will be long at the lowfrequencies anticipated, and consequently the excitation power is low,typically a few watts.

The NMR coil 360 can be realized in a number of ways. The simplest is asolenoid, which has a high efficiency, and is a preferred geometry atlow frequencies. The quality factor of the coil is maximized by the useof Litz wire. The coil may be tuned (double tuned) or non-tuned duringexcitation. Likewise, the coil can be tuned or non-tuned in receptionmode. The switching means 365 can be a mechanical relay or passivecircuitry.

The receiver-part of the spectrometer is convenient for controlling andoptimizing the spectrometer performance, but is not crucial for theproduction of the contrast agent. It can also be used for polarizationmeasurement. An important part of the receiver-chain is the low noiseamplifier 380, which is needed for enabling optimal performance of thespectrometer. The amplifier should have a sufficiently large bandwidthand low noise (e.g. less than 3dB). In the case that the coil is tunedthe bandwidth may be increased by negative feedback (active damping).

The amplified signal is digitalized directly and filtered to anappropriate bandwidth by an A/D converter 385, preferably provided as aPC card. Interpolation and decimation reduce the signal frequency andnumber of sampling points. A Fast Fourier Transform (FFT) is applied tothe signal in order to calculate the NMR spectrum.

Preferably, most of the functions are integrated in a PC environment, inparticular the frequency generation and sampling. The automatic shimmingand field locking can also be handled from the PC.

A method according to the invention, advantageously utilizing the abovedescribed apparatus, for increasing the polarization of a contrastagent, will now be described. The method causes a transfer of protonspin order to a J-coupled long-T₁ nucleus by rf-pulse sequences. Thepulses are applied to the contrast agent prior to injection into forexample a patient. The method is generally applicable for use with alarge variety of substrates, but the pulse sequence will be specific forthe substrate and the stationary magnetic field, B₀. The method will beexemplified with a molecule containing only one none-zero spin nucleus,¹³C with S=½, which is hydrogenated with para-hydrogen and preferably inaqueous solution. It should be noted that the method according to thepresent invention is not limited to this example. The method of usingrf-pulse sequences for spin order transfer is applicable to anymolecules with spin ½hetero nucleus.

A prerequisite for the method is that the hydrogenation catalyst worksby such a mechanism that the two hydrogen atoms from one molecule ofpara-hydrogen are delivered to one molecule of substrate to produce onemolecule of product. These protons pairs are then produced in a singletspin state, which corresponds to a high degree of order, and therf-pulse sequence is intended to transfer this order to carbonpolarization.

The hydrogenation is typically performed in the earth field, and thehydrogenation will occur at different times for the individualmolecules. This will result in a system finally being in a statisticalsteady-state characterized by various populations of the energy statesof the spin system, corresponding to the loss of the interstate initialcorrelations, and the subsequent loss of some of the initial order. Themethod according to the invention uses spin excitations by the way ofradiofrequency pulses (rf-pulses) on either spin species, i.e. thefrequency is adjusted to correspond to the Larmor-frequency of thenuclei. This is performed in a magnetic field sufficiently low such thatthe proton spins are in the so-called strong coupling regime, that is,the difference of the carbon-protons couplings and/or the difference ofproton resonance frequencies due to different chemical shifts are notlarge compared with the inter-proton interactions. This has twoimportant advantages. Firstly, there is no need to use a large magneticfield for the order-to-polarization step, and secondly the veryexistence of a strong inter-spin coupling is the root of the efficientmanipulation of the spin state of the system.

In the embodiments of the present invention the notations definingangles of the applied fields refers to a rotating frame and are thenotations commonly used in the discipline of NMR. The phrases “pulse oncarbon”, “pulse on hydrogen” etc should be understood as a rf-pulse withthe frequency adapted to the nucleus and the stationary field as tocorrespond to the Larmor frequency of the nucleus.

Two alternative embodiments of the invention directly applicable in thecase of the carbon nucleus having only two adjacent protons will bedescribed. These prerequisites hold for e.g.2,3-dideuterosuccinate-1-¹³C. The case when more than two protons arecoupled to the carbon nuclues is more complicated than the former onegiving a much more complex structure of the quantum energy states. Thisrequires parameters such as the delay times between pulses and theangles of the applied fields to be adjusted by optimization, either bycomputer simulation or by experiments.

The pulse sequence according to the first embodiment comprises twoconsecutive main parts:

The first part is a preparation of the state of the system starting fromthe steady-state density matrix at the end of the hydrogenation, through180°-pulses followed by evolution periods, and the second part isproduction of a net spin S (carbon) polarization by way of non-180°pulses (initially 90°) and evolution periods. In principle only thespins S are excited, but in practice one may apply 180° pulses on thespins I (protons) simultaneously with 180° pulses on the spins S toreduce the effects of field inhomogeneities.

The steady-state density matrix, σ₃₃, after hydrogenation is given bythe projection of the density matrix$\sigma = {\frac{1}{8}\left( {1 - {4{\hat{I}}_{1}{\hat{I}}_{2}}} \right)}$on the eigen states of the Hamiltonian (rotating frame),H=J₁₂Î₁Î₂+J₁₃I₃I_(1x)S_(z)+J₂₃I_(2z)S_(z), i.e.$\sigma_{ss} = {\frac{1}{8}{\left( {1 - {4\left( {{I_{1x}I_{2x}} + {{pS}_{x}\left( {I_{1x} - I_{2x}} \right)} + {q\left( {{I_{1x}I_{2x}} + {I_{1y}I_{2y}}} \right)}} \right)}} \right).}}$The coefficients p and q depend on the scalar couplings, J_(ij), anddetermine the theoretical limit of the obtainable polarization.

The first main part consists of transforming the steady-state densitymatrix above to a state that as closely as possibly resemblesS_(z)(I_(1y)I_(2x)−I _(1x)I_(2y))≡S_(z)K_(Y), i.e. to optimize the(scalar) constant in front of this operator. This is achieved by aseries of 180° pulses and subsequent evolution periods.

The second main part of the sequence starts with a 90_(y)(S) pulse thatconverts S_(z)K_(y) into S_(x)K_(y). The subsequent evolution transformsthis state into 2S_(x)K_(y)→exp(−iHt)2S_(x)K_(y)exp (iHt) which is alinear combination of S_(y,) 2K_(y)S_(x) and 2K_(z)S_(x)$\left( {{{where}\quad K_{z}} \equiv {\frac{1}{2}\left( {I_{1z} - I_{2x}} \right)}} \right).$The coefficients in front of these terms oscillate, and the duration ofthe evolution will be chosen so that the term containing S_(y) ismaximized (at the same time 2K_(z)S_(x) vanishes). Then a 90_(x)(S)pulse is applied that produces a linear combination of S_(z) and2K_(y)S_(x). Another (identical) evolution time gives a linearcombination of the terms (S_(z)+CS_(y)) and 2K_(y)S_(x). A pulse ofangle φ=arctan(C), φ_(x)(S), will give the terms S_(z) and2K_(y)S_(x)(the last term is unaffected by a pulse on S around O_(x)).Neglecting relaxation a subsequent evolution will not affect S_(z),whereas the term 2K_(y)S_(x) transforms as above. Repeating the aboveprocedure, changing the angle φ accordingly, increases the polarizationof carbon (i.e. S_(z)).

A realization of the first embodiment of the method according to theinvention will be described with reference to the flowchart of FIG. 4.The realization of the first embodiment comprises the steps of:

-   400: Applying a stationary magnetic field B₀.-   410: Hydrogenation of the substrate with para-hydrogen in the    presence of the magnetic field B₀.-   420: Applying a series of 180°_(x) pulses followed by delays t_(i)    on carbon. The delays after each pulse will typically differ. The    delays t_(i) are typically in the order of 1-100 ms. The number of    repetitions are typically one or two but may be more. The purpose of    this step is to bring the system into a state consisting of a zero    quantum coherence involving the two protons and the carbon. The    determination of delays and number of repetitions can be made    analytically (if having one carbon and two protons) or by numerical    methods (more spins) and involves maximizing the term    S_(z)(I_(1y)I_(2x)−I_(1x)I_(2y)), in which S refers to the spin    operator of carbon and I₁ and I₂ refer to the spin operators of the    two protons. Step 420 corresponds to the first main part of the    embodiment.-   430: Applying a 90°_(y) pulse on carbon.-   440: Waiting for t/2 s.-   450: Optionally applying simultaneous 180°_(x) pulses on hydrogen    and carbon in order to compensate for the effect of field    inhomogeneities (spin echo pulses).-   460: Optionally waiting for t/2 s.-   470: Applying a pulse φ_(x) on carbon, the angle φ_(x) may be    different in each step and as previously described determined by    analytical calculation or by numerical simulation.-   480: Repeating steps 440 to 470 m times. Steps 440-470 gives a    progressive build up of carbon polarization in the direction of the    external field axis. The number of repetitions, m, is typically 1-5.    The steps 430-480 corresponds to the second main part of the    embodiment.

The first embodiment is characterized in that the pulses substantiallyeffecting the polarization are applied only to the carbon nucleus, i.e.with a frequency corresponding to the Larmor frequency of carbon. Thestep involving pulses on both carbon and hydrogen is only taken tocompensate for experimental imperfections. Depending on the molecule, acarbon spin polarization between 50% and 85% is typically achieved. Thetotal duration of the polarization process is about one to severalhundred milliseconds. Thus, the method is fast which can be crucialbecause of the loss of polarization due to spin relaxation.

The second embodiment comprises three consecutive main parts. The firstmain part transforms the steady-state density matrix (see above) to astate that as closely as possibly resembles${{\frac{1}{8}\left( {1 - {4\left( {{I_{1z}I_{2z}} - {I_{1x}I_{2x}} - {I_{1y}I_{2y}}} \right)}} \right)} \equiv {\frac{1}{8}\left( {1 - {4\left( {{I_{1z}I_{2z}} - K_{x}} \right)}} \right)}},$i.e. optimizes the (scalar) constant in front of the operator K_(z)(optimally equal to one). This is accomplished by a series of 180°pulses and subsequent evolution periods of equal duration. At the end ofthis part the density matrix is proportional to (disregarding smallterms and the unimportant constant term) I_(1z)I_(2x)+cK_(x) (c beingclose to unity). The second main part starts with a 90_(y)(I) pulse thatproduces the term (I_(1x)I_(2x)−I_(1y)I_(2y)). A subsequent evolutionperiod gives the term S_(z)(I_(1y)I_(2x)+I_(1x)I_(2y)). The third mainpart starts with a 90_(y)(S) pulse transforming S_(z)→S_(x) and a90_(φ)(I) pulse, with a suitable phase, φ, that transforms(I_(1y)I_(2x)+I_(1x)I_(2y)) into (I_(1z)I_(2z)−I_(1y)I_(2y)). The usefulpart of the density operator is now 2S_(x)I_(1z)I_(2z). An evolutionperiod produces the useful part S_(x) (neglecting terms that are of nointerest) which is finally transformed to S_(z) by a 90_(y)(S) pulse.

A realization of the second embodiment of the method according to theinvention will be described with reference to the flowchart of FIG. 5.The realization of the second embodiment comprises the steps of:

-   500: Applying a stationary magnetic field B₀.-   510: Hydrogenation of the substrate with para-hydrogen in the    presence of the magnetic field B₀.-   520: Applying a series of 180°_(x) pulses followed by delays t₁ on    carbon. The number of repetitions are typically one or two but may    be more. The purpose of this step is to bring the system into a    state consisting of a two proton zero quantum coherence. The    determination of delays and number of repetitions can be made    analytically (one carbon, two protons) or by numerical methods (more    spins) and involves maximizing the factor    (I_(1x)I_(2x)−I_(1y)I_(2y)). Step 520 corresponds to the first main    part of the second embodiment.-   530: Applying a 90°_(y) pulse on hydrogen, which results in a    two-proton-double quantum coherence.-   540: Waiting for t₂/2 s. During this time the two-proton-double    quantum coherence transforms into a three-spin coherence involving    the carbon spin. Steps 530 and 540 corresponds to the second main    part of the second embodiment.-   550: Optionally applying simultaneous 180°_(x) pulses on hydrogen    and carbon in order to compensate for the effect of field    inhomogeneities (spin echo pulses).-   560: Optionally waiting for t₂/2 s.-   570: Applying simultaneous 90°_(y) and 90°φ pulses on carbon and    hydrogen, respectively, for producing a transverse carbon    polarization.-   580: Waiting for t₃/2 s.-   585: Applying simultaneous 180°_(x) pulses on carbon and hydrogen.-   590: Waiting for t₃/2 s.-   595: Applying a −90°_(y) pulse on carbon, for transforming the    transverse carbon polarization into a longitudinal polarization,    which is parallel to the applied field. Steps 570-595 corresponds to    the third main part of the second embodiment.

It should be noted that in the second embodiment, contrary to the firstembodiment, rf-pulses directly effecting the polarization are applied toboth the carbon and hydrogen nuclei. The polarizations achievable by thesecond embodiment are typically between 60% and 98%, depending on themolecule. For most instances, they are larger than in the firstembodiment, but the pulse sequence is in most cases more complex and theduration of the polarization sequence is in general longer. Hence, theloss due to spin relaxation during the increased process duration has tobe considered. Which of the embodiments to use under practicalconditions is thus decided based on these factors and on the capabilityof the equipment. The decision is typically “case by case”.Implementation examples and simulated results are enclosed in AppendixA. As appreciated by the one skilled in NMR and MRI the tools andpractices utilized in determining the pulse sequences in NMR and MRIapplications can also be used as a guidance for determination of thepulse sequences used for producing contrast agent according to thepresent invention.

It is conceivable to use pulses that are modulated in amplitude, phaseor frequency or any combination of those. Alternatively, the individualpulses can be substituted with composite pulses (i.e. sequences ofphase-shifted rf pulses). This provides better phase-control over awider bandwidth. This gives better results in practice and relaxes thedemands on precision of the hardware. Examples of composite pulses canbe found in: “Band-Selective Radiofrequency Pulses”, Geen, H. Freeman,R. J. Magn. Res. 93, 93-141(1991), “Composite Pulses” Levitt, M.Progress in NMR spectroscopy, 18, 61-122 (1986), “Composite Pulseswithout Phase Distortion” Tycko, R. J. Magn. Res. 61, 90-101 (1985) and“Broadband, Narrowband, and Passband Composite Pulses for Use inAdvanced NMR Experiments” Wimperis, S. J. Magn. Res. Ser A109, 221-231(1994).

In both the described embodiments the delay times, t_(i), are typicallyin the order of 1-100 ms. The amplitude of the rf-fields is typically1-100 μT, the duration of the pulses follows from the rf-amplitude andthe frequency adapted to the Larmor frequency of the target nucleus. Thestationary magnetic field B₀, applied in step 400 and 500 respectively,is preferably 0.1-100 mT, and more preferably 1-10 mT.

The method according to the invention will facilitate the production ofhighly polarized contrast agent. The properties of the contrast agentmay further be improved if the relaxation rate of the spin system isreduced. One way of reducing the relaxation rate of the spin system,i.e. extending the useful lifetime of the final contrast agent, is bydecoupling of the former para-hydrogen protons during the process ofhydrogenation. The apparatus and the method according to the inventionof subjecting the contrast agent to rf-pulses, may advantageously beused also to reduce the relaxation of the spin system.

During the process of hydrogenation a substantial relaxation of the spinsystem occurs. The relaxation takes place both when the para-hydrogen isattached to the catalyst in the intermediate state and later, when it istransferred to the molecules of the contrast agent. The relaxationresults in a reduced PHIP effect and impairs the performance of thecontrast agent. During the hydrogenation different molecules will behydrogenated at different times and the former para-hydrogen protonswill no longer be in the singlet state. Intramolecular dipole-dipolecoupling will cause relaxation of the spin system. The dipole-dipoleinteraction contribution to the relaxation can be significantly reducedby forcing the protons to remain in the singlet state. From a quantummechanical consideration of the spin system it can be realized that, byletting the process of hydrogenation take place in the presence of botha rf-field and a static magnetic field, the protons can be forced toremain in their singlet state. Hence the relaxation can be significantlyreduced.

In order to utilize the method of exposure to rf-field and magneticfield during hydrogenation the reactor 210 has to be equipped with meansfor producing the fields. In addition the material of the reactor 210has to be transparent both to the rf irradiation and the static magneticfield. Preferably a non-conducting, i.e non-metallic material is used,for example glass-fibre reinforced epoxy. This material is also capableof withstanding the substantial pressure used during the process. Theexternal static magnetic field and the rf-fields can preferably beprovided by a magnet system similar to that of the magnetic treatmentunit 240 described above. Alternatively the external magnetic field maybe provided by a solenoid coil. The required field strength is in theorder of 1 mT and the requirement of homogeneity is fairlymoderate—inhomogeneities of up to ±6% are acceptable. The rf-field maypreferably be provided by crossed orthogonal saddle coils, which canproduce circularly polarized rf-fields as well as more complexdecoupling sequences. The frequency of the rf-field should correspond tothe Larmor frequency of the protons, e.g. 42.6 kHz if the external fieldis 1 mT ( ${v = \frac{\gamma\quad B_{0}}{2\quad\pi}},$where ν is frequency of the rf-field, γ the gyromagnetic ratio and B₀the magnetic field). The intensity of the rf-field should preferably beoptimized for each case. A suitable rf-field amplitude will typically beapproximately 50 μT, with an acceptable inhomogeneity of the order of 10μT. The generation and control of the required fields are well-known inthe area of NMR and the chosen implementation can be modified in manyways as appreciated by those skilled in the art. All the magnetic fieldtreatments are preferably done with the same magnet system and in thesame chamber. Thus, in this preferred embodiment of the apparatus, thereactor 210 and the magnetic treatment unit 240 are combined into oneunit.

The embodiments of the method according to the invention described abovemay be modified to also include the magnetic treatment for reducing therelaxation by letting the step of hydrogenation (step 400, 500) to beperformed in the presence of a combined external magnetic field and anrf-field. The step in this preferred embodiment being:

-   400, 500: Hydrogenation of the substrate with para-hydrogen,    performed in the presence of an external stationary magnetic field    and an external rf-field.

The method of exposing the contrast agent to a combined externalmagnetic field and an rf-field during the hydrogenation may be combinedalso with methods of spin order transfer other than the here describedmethod utilizing series of rf-pulses. For example, the method of usingfield cycling schemes described in WO 00/71166, may be improved if acombined external magnetic field and an rf-field is present during thehydrogenation.

As the magnetic treatment unit 240 in a preferred embodiment of theinvention has all the facilities of a two-channel NMR spectrometer, itcan be advantageously used for characterizing and checking the qualityof the produced contrast agent. The ability of characterizing, forexample measuring the degree of polarization in the produced contrastagent, is valuable in developing and fine-tuning the pulse sequences ofthe above-described embodiments. This is obviously most useful in thecases when the parameters of the pulse sequences have to be establishedfrom experiments, but also if the parameters are determined analyticallyor by computerized optimization, the degree of polarization may beincreased by fine-tuning parameters, for example the delay times,experimentally.

The method of the invention is preferably implemented by means of acomputer program product comprising the software code means forperforming the steps of the method by controlling parts of the apparatusaccording to the invention. The computer program product is typicallyexecuted on the computer 330. The computer program is loaded directly orfrom a computer usable medium, such as a floppy disc, a CD, the Internetetc.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the inventive concept, and all such modifications aswould be obvious to one skilled in the art are intended for inclusionwithin the scope of the following claims.

Appendix A

Exemplary Pulse Sequences:

Pulses given in degrees with phase as subscript, delays as time t withindex as subscript. Sequence 1-2 corresponds to the first embodiment ofthe invention, sequence 3 to the second embodiment and sequence 4 is aspecial case related to double hydrogenation. Sequence 1a: ¹³C180_(x)-t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x) ¹H ------------------------180_(x) Sequence 1b: ¹³C180_(x)-t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x)-t₃/2-180_(x)-t₃/2-φ2_(x)¹H  ------------------------180_(x)-t₃------------180_(x) Sequence 1c:¹³C180_(x)-t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x)-t₃/2-180_(x)-t₃/2-φ2_(x)-t₃/2-180_(x)-t₃/2-φ3_(x)¹H ------------------------180_(x)-t₃------------180_(x)-t₃------------180_(x)Sequence 1d: ¹³C180_(x)-t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x)-t₃/2-180_(x)-t₃/2-φ2_(x)-t₃/2-180_(x)-t₃/2-φ3_(x)t₃/2-180_(x)-t₃/2-φ4_(x)¹H ------------------------180_(x)-t₃------------180_(x)-t₃------------180_(x)-t₃------------180_(x)Sequence 2a: ¹³C180_(x)-t₃-180_(x)t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x) ¹H --------------------------------180_(x) Sequence 2b: ¹³C180_(x)-t₃-180_(x)t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x)-t₃/2-180_(x)-t₃/2-φ2_(x)¹H  --------------------------------180_(x)-t₃------------180_(x)Sequence 2c: ¹³C180_(x)-t₃-180_(x)t₁-180_(x)-t₂-90_(y)-t₃/2-180_(x)-t₃/2-φ1_(x)-t₃/2-180_(x)-t₃/2-φ2_(x)-t₃/2-180_(x)-t₃/2-1003_(x) ¹H --------------------------------180_(x)-t₃------------180_(x)-t₃------------180_(x)Sequence 3: ¹³C(180_(x)-t₁)_(n)-t₂/2------180_(x)-t₂/2-90_(y)-t₃/2-180_(x)-t₃/2-90_(y)¹H  -----------90_(y)-t₂/2-180_(x)-t₂/2-90φ-t₃/2-180_(x) Sequence 4: ¹³C180_(x)-t₁-90_(y)-t₂/2-180_(x)-t₂/2-90_(x) ¹H  ----------------180_(x)

EXAMPLES OF PREDICTIONS

Contrast Agent: Maleic Acid

J_(HH=)10.65 Hz, J_(H1C)=15.5 Hz, J_(H2C)=0.5 Hz

Pulse Sequence 1a, B₀=5 mT, t₁=20.76 ms, t₂=33.91 ms, t₃=38.39 ms,φ1=90°:

Polarization P=76.98%, Total duration=93.06 ms.

Pulse Sequence 1b, B₀=5 mT, t₁=20.76 ms, t₂=33.91 ms, t₃=38.39 ms,φ1=90°, φ2=−18.62°:

Polarization P=−81.23%, Total duration=131.45 ms.

Pulse Sequence 1c, B₀=5 mT, t₁=20.76 ms, t₂=33.91 ms, t₃=38.39 ms,φ1=90°, φ2=−18.62°, φ3=6.14°:

Polarization P=81.7%, Total duration=169.83 ms.

Pulse Sequence 3, B₀=5 mT, n=2, t₁=38.4 ms, t₂=31.25 ms; t₃=69 ms:

Polarization P=85.4%, Total duration=177.0 ms.

Contrast Agent: Succinic Acid (2.3-D₂) J_(HHviC)=7 Hz, J_(H1C)=7 Hz,J_(H2C)=0 Hz, ΔδH=0 ppm

Pulse Sequence 1a, B₀=5 mT, t₁=44.09 ms, t₂=46.64 ms, t₃=63.89 ms,φ1=90°:

Polarization P=71.55%, Total duration=154.62 ms.

Pulse Sequence 1b, B₀=5 mT, t₁=44.09 ms, t₂=46.64 ms, t₃=63.89 ms,φ1=90°, φ2=−30.96°:

Polarization P=−83.45%, Total duration=218.51 ms.

Pulse Sequence 1c, B₀=5 mT, t₁=44.09 ms, t₂=46.64 ms, t₃=63.89 ms,φ1=90°, φ2=−30.96°, φ3=17.16°:

Polarization P=87.33%, Total duration=282.4 ms.

Pulse Sequence 1d, B₀=5 mT, t₁=44.09 ms, t₂=46.64 ms, t₃=63.89 ms,φ1=90°, φ2=−30.96°, φ3=17.16°, φ3=−10.04°:

Polarization P=−88.69%, Total duration=346.29 ms.

Pulse Sequence 3, B₀=5 mT, n=3, t₁=63.9 ms, t₂=71.4 ms, t3=135.4 ms:

Polarization P=93.2%, Total duration=398 ms.

Contrast Agent: O-Acetyl lactic acid (3.3-D₂) J_(HH)=7.1 Hz, J_(H1C)=7.3Hz, J_(H2C)=2.4 Hz, ΔδH=3.2 ppm

Pulse Sequence 2a, B₀=5 mT, t₁=40.07 ms, t₂=58.12 ms, t₃=66.57 ms,φ1=90°:

Polarization P=57.6%, Total duration=231.33 ms.

Pulse Sequence 2b, B₀=5 mT, t₁=40.07 ms, t₂=58.12 ms, t₃=66.57 ms,φ1=90°, φ2=−38.2°:

Polarization P=−72.06%, Total duration=297.9 ms.

Pulse Sequence 2c, B₀=5 mT, t₁=40.07 ms, t₂=58.12 ms, t₃=66.57 ms,φ1=90°, φ2=−38.2°, φ3=25.96°:

Polarization P=78.80%, Total duration=364.47 ms.

Pulse Sequence 3, B₀=5 mT, n=4, t₁=63.9 ms, t₂=71.4 ms, t3=135.4 ms:

Polarization P=93.07%, Total duration=424.7 ms.

Special Case: Succinic Acid, Produced from Acetylene Dicarboxylic Acidby Two Parahydrogen Reactions in Rapid Sequence: J_(HHvic)=7 Hz,J_(HHgem)=14 Hz, J_(H1C)=7 Hz, J_(H2C)0 Hz, ΔδH=0 ppm

Pulse sequence 4, t₁=28.43 ms, t₂=56.9 ms:

Polarization P=57%, Total duration=85.3 ms.

1. A method for producing MR contrast agent, the method comprising thesteps of: obtaining (100) a solution in a solvent of a hydrogenatable,unsaturated substrate compound and a catalyst for the hydrogenation of asubstrate compound, wherein the substrate compound comprises imagingnuclei; hydrogenating (110) the substrate with hydrogen gas (H₂)enriched in para-hydrogen (p-¹H₂) to form a hydrogenated contrast agent;exposing (120) the contrast agent to a oscillating magnetic field incombination with a stationary magnetic field for enhancing thecontrasting effects of the contrast agent adapted for use in an MRapplication.
 2. The method according to claim 1 wherein the oscillatingmagnetic field is oscillating with a frequency within the region ofradio frequencies (e.g from around 10 Hz to several GHz).
 3. The methodaccording to claim 1 wherein the oscillating magnetic field isoscillating with a frequency in the interval 5 kHz to 500 MHz.
 4. Themethod according to claim 2 wherein the step of exposure to theoscillating magnetic field in combination with the stationary magneticfield is performed during the step of hydrogenation, wherein the step ofexposure is performed for reducing the relaxation of the spin system ofthe contrast agent, whereby the contrasting effects of the contrastagent is enhanced.
 5. The method according to claim 2 wherein the stepof exposure to the oscillating magnetic field in combination with thestationary magnetic field is to be performed after the step ofhydrogenation, the step of exposure is performed for enhancing thedegree of polarization of an imaging nuclei of the contrast agent,whereby the contrasting effects of the contrast agent is enhanced. 6.The method according to claim 5 wherein the step of exposure to theoscillating magnetic field in combination with the stationary magneticfield comprises exposing the contrast agent to at least one series ofpulses of the oscillating magnetic field (rf-pulse).
 7. The methodaccording to claim 6 wherein the exposing step comprises: applying (420)a first series of pulses of the Larmor frequency of the imaging nucleiof the hydrogenated contrast agent and delays between the pulses, thefirst series adapted to bring the system into a state consisting of azero quantum coherence involving the protons and the imaging nuclei;applying (430-480) a second series of pulses of the Larmor frequency ofthe imaging nuclei of the hydrogenated contrast agent and delays betweenthe pulses, the second series adapted to give a progressive build up ofcarbon polarization in the direction of the external field axis.
 8. Themethod according to claim 6 wherein the exposing step comprises:(a)—applying (420) a series of 180°_(x) pulses followed by delays(t_(i))on the imaging nuclei; (b)—applying (430) a 90°_(y) pulse on carbon;(c)—waiting (440) for t/2 s; (d)—Optionally applying (450) simultaneous180°_(x) pulses on hydrogen and the imaging nuclei in order tocompensate for the effect of field inhomogeneities; (e)—Optionallywaiting (460) for t/2 s; (f)—applying (470) a pulse with an angle φ_(x)on the imaging nuclei; (g)—Optionally repeating steps c to f to producea progressive build up of the imaging nuclei polarization in thedirection of the external field axis, wherein the angle φ_(x) may bedifferent in each repetition.
 9. The method according to claim 6 whereinthe exposing step comprises: applying (520) a first series of pulses ofthe Larmor frequency of the imaging nuclei of the hydrogenated contrastagent and delays between the pulses, the first series adapted to bringthe system into a state consisting of a zero quantum coherence involvingthe protons and the imaging nuclei; applying (530-540) a second seriesof pulses and delays between the pulses comprising of pulses of theLarmor frequency of the imaging nuclei of the hydrogenated contrastagent alternated with pulses of the Larmor frequency of the hydrogen ofthe hydrogenated contrast agent, the second series adapted to transforma two-proton-double quantum coherence into a three-spin coherenceinvolving the spins of the imaging nuclei; applying (570) simultaneous90°_(y) and 90°_(φ) pulses on the imaging nuclei and hydrogen,respectively, adapted for producing a transverse polarization of theimaging nuclei.
 10. The method according to claim 6 wherein the exposingstep comprises: applying (520) a series of 180°_(x) pulses followed bydelays t₁ on the imaging nuclei; applying (530) a 90°_(y) pulse onhydrogen; waiting (540) for t₂/2 s; optionally applying (550)simultaneous 180°_(x) pulses on hydrogen and the imaging nuclei in orderto compensate for the effect of field inhomogeneities; optionallywaiting (560) for t₂/2 s; applying simultaneous (570) 90°_(y) and90°_(φ) pulses on the imaging nuclei and hydrogen; waiting (580) fort₃/2 s; applying (585) simultaneous 180°_(x) pulses on the imagingnuclei and hydrogen; waiting (590) for t₃/2 s; applying (595) a −90°_(y)pulse on carbon.
 11. Method according to claim 7 wherein one or more ofthe radiofrequency pulses is either composite or modulated in amplitude,phase or frequency or any combination thereof.
 12. Apparatus forproducing MR contrast agent, the apparatus comprising a magnetictreatment unit (240) adapted for magnetic treatment of the contrastagent, characterised in that the magnetic treatment unit (240) comprisesmeans for producing an oscillating magnetic field and means forproducing a stationary magnetic field.
 13. Apparatus according to claim11 wherein said magnetic treatment unit (240) is combined with ahydrogenation reactor
 210. 14. Apparatus according to claim 12 whereinsaid magnetic treatment unit (240) comprises essentially the magneticsystem of a NMR spectrometer.
 15. Apparatus according to claim 14wherein said magnetic system of a NMR spectrometer additionally are usedfor analyzing the produced contrast agent with NMR spectroscopy. 16.Apparatus according to claim 12 wherein said magnetic treatment unit(240) comprises a Helmholtz pair (360) for producing the stationarymagnetic field and a NMR coil (360) for producing the oscillatingmagnetic field.
 17. A computer program product directly loadable intothe internal memory of a processing means within a processing unit forcontrolling the method and apparatus for producing MR contrast agent,comprising the software code means adapted for controlling the steps ofclaim
 1. 18. A computer program product stored on a computer usablemedium, comprising a readable program adapted for causing a processingmeans, in a processing unit for controlling the method and apparatus forproducing MR contrast agent, to control an execution of the steps ofclaim 1.